staging: rtl8712: Replace explicit NULL comparison

Replace explicit NULL comparison with ! operator.
Found with Coccinelle.

@@
expression e;
@@
- e == NULL
+ !e

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>